The present invention relates to a magnetic tunnel junction element and a magnetic memory using the same.
Recently, the application of magnetic tunnel junction (MTJ) elements to playback magnetic heads for hard disk drives (HDDs) and to magnetic memories has been considered and discussed because the MTJ elements provides a higher output, compared with conventional anisotropic magnetoresistive (AMR) elements and giant magnetoresistive (GMR) elements.
In particular, magnetic memories, which are solid state memories having no operating parts similar to semiconductor memories, are more useful than the semiconductor memories because of the following characteristics of their own: the information stored therein is not lost even if electric power is disconnected; the number of repetitive rewrites is infinite, namely, an infinite endurance is provided; there is no risk of destroying the recorded contents even if exposed to radioactive rays, etc.
As an example of the constitution of conventional MTJ elements, the one according to the teaching of JP-A-9-106514 is shown in FIG. 12.
The MTJ element in FIG. 12 is constituted of an antiferromagnetic layer 41, a ferromagnetic layer 42, an insulating layer 43, and a ferromagnetic layer 44. As the antiferromagnetic layer 41, an alloy such as FeMn, NiMn, PtMn or IrMn is used. As the ferromagnetic layers 42 and 44, Fe, Co, Ni or an alloy thereof is used. Further, as the insulating layer 43, various oxides and nitrides are being studied, and it is known that the highest magnetoresistance (MR) ratio is obtained when using an Al2O3 film.
In addition to this, there has been proposed an MTJ element with the antiferromagnetic layer 41 eliminated to utilize a difference in coercive force between the ferromagnetic layers 42 and 44.
FIG. 13 shows the principle of operation of the MTJ element having the constitution shown in FIG. 12 where the MTJ element is used for a magnetic memory.
The magnetizations of both the ferromagnetic layers 42 and 44 are within a film surface and have effective uniaxial magnetic anisotropy such that the magnetizations of these layers are parallel or antiparallel with each other. The magnetization of the ferromagnetic layer 42 is fixed substantially in one direction by the exchange coupling with the antiferromagnetic layer 41, and a recorded content is stored according to the direction of magnetization of the ferromagnetic layer 44.
The resistance of the MTJ element differs depending on whether the magnetization of the ferromagnetic layer 44 to serve as a memory layer is parallel or antiparallel to the direction of magnetization of the ferromagnetic layer 42. Utilizing the difference in magnetic resistance, information is read from the MTJ element by detecting its magnetic resistance. On the other hand, information is written to the MTJ element by changing the direction of magnetization in the ferromagnetic layer 44 using a magnetic field generated by current lines placed in the vicinity of the MTJ element.
In the MTJ element having the above constitution, the ferromagnetic layers 42 and 44 are magnetized parallel to the layer surfaces, and thus magnetic poles are generated at opposite end portions of these layer surfaces.
For increasing the packing density or integration degree in the magnetic memory, the size reduction of the MTJ elements is required. However, with the size reduction of the elements, an influence of the diamagnetic field due to the magnetic poles at the opposite end portions becomes greater.
Since the ferromagnetic layer 42 is exchange-coupled with the antiferromagnetic layer 41, the influence of the diamagnetic field upon the ferromagnetic layer 42 is small. Further, it is possible to substantially eliminate magnetic poles at the end portions by constituting the ferromagnetic layer 42 of two ferromagnetic layers that are antiferromagnetically coupled with each other as disclosed in U.S. Pat. No. 5841692.
However, as to the ferromagnetic layer 44 to serve as a memory layer, a similar technique cannot be applied. Thus, with finer patterns, the magnetization of the ferromagnetic layer 44 becomes unstable due to the influence of the magnetic poles at the end portions, which makes it difficult for the ferromagnetic layer 44 to hold a recorded content.
Japanese publication JP-A-11-163436 discloses that in order to realize an increase of output voltage, three ferromagnetic layers and two insulating layers are alternately laid on one another to thereby form two magnetic tunnel junctions in an MTJ element. This MTJ element provides an output that is about twice of that of an MTJ element having a single magnetic tunnel junction. However, since the three ferromagnetic layers are magnetized along their layer surfaces, there arises a problem similar to the above-described problem inherent in the MTJ element of FIG. 12.
Further, with the reduction in the area of a memory cell in a magnetic thin film memory, it becomes impossible to ignore the diamagnetic field (self-demagnetizing field) that will occur inside the magnetic layer. Due to this, the magnetization of the magnetic layer that stores information is not fixed in one direction, thus becoming unstable. As a solution to this problem, Japanese Patent Publications JP-A-10-302456 and JP-A-10-302457 disclose to provide a laminated film consisting of a first magnetic layer, a non-magnetic layer and a second magnetic layer, which all together form a memory cell, with a third magnetic layer at both sides of the laminated film, so as to form a closed magnetic circuit surrounding the non-magnetic layer by the first, second and third magnetic layers when an external magnetic field is zero.
A magnetic tunnel junction element in which an extremely thin insulating film is formed as the non-magnetic layer of the laminated film exhibits a great change in magnetoresistance, and thus the element is regarded as a promising memory cell having a high output. In this case, the third layer is required to be formed of an insulating material. However, an insulative magnetic layer having a coercive force low enough to realize a closed magnetic circuit structure is extremely difficult to form with a currently available technique. Accordingly, it is unrealistic.
An object of the present invention is to provide a magnetic tunnel junction element that enables a magnetization state recorded in the memory layer to exist stably even if finer patterns are used, and also to provide a magnetic memory using such a magnetic tunnel junction element.
In order to accomplish the above object, a magnetic tunnel junction element according to an aspect of the present invention comprises:
a first magnetic layer and a second magnetic layer acting as a memory layer;
a first insulating layer disposed between the first and second magnetic layers; and
a third magnetic layer provided on a side of the second magnetic layer opposite from the first insulating layer so as to form a closed magnetic circuit together with the second magnetic layer.
According to the present invention, the influence of the end-portion magnetic poles is reduced. Therefore, even if a finer pattern is used for the magnetic tunnel junction (MTJ) element, the magnetized state is retained stably. Further, because the ferromagnetic layer to serve as a memory layer forms a closed magnetic circuit structure, the MTJ element of the present invention becomes stable to an external leakage magnetic field.
Further, since the MTJ element of the present invention can stably retain the magnetized state thanks to the reduced influence of the magnetic poles at the end portions even if a finer patter therefor is used, it is possible to realize a magnetic memory having a higher integration degree and consuming less power.
In one embodiment, the third magnetic layer is joined to the second magnetic layer at opposite ends thereof directly or through fourth magnetic layers, with a central portion of the third magnetic layer spaced from the second magnetic layer.
A lead wire may be disposed within a gap defined between the second magnetic layer and the central portion of the third magnetic layer, through an insulating layer.
The MTJ element may further include a first antiferromagnetic layer in contact with a face of the first magnetic layer opposite from the first insulating layer, wherein the first antiferromagnetic layer is exchange-coupled with the first magnetic layer.
The first magnetic layer may be constituted of at least two ferromagnetic sub-layers which are antiferromagnetically coupled with each other through a metal layer.
In one embodiment, the MTJ element further includes:
a fifth magnetic layer formed on a side of the third magnetic layer opposite from the first magnetic layer; and
a second insulating layer disposed between the third and fifth magnetic layers.
This MTJ element may further include:
a first antiferromagnetic layer in contact with a face of the first magnetic layer opposite from the first insulating layer, said first antiferromagnetic layer being exchange-coupled with the first magnetic layer; and
a second antiferromagnetic layer in contact with a face of the fifth magnetic layer opposite from the second insulating layer, said second antiferromagnetic layer being exchange-coupled with the fifth magnetic layer.
When the MTJ element includes the first and second antiferromagnetic layers, it is preferable that a temperature at which the exchange coupling of the first antiferromagnetic layer with the first ferromagnetic layer disappears is different from a temperature at which the exchange coupling of the second antiferromagnetic layer with the fifth ferromagnetic layer disappears.
A magnetic memory according to another aspect of the present invention uses the MTJ element having the above-described structure as a memory cell. Accordingly, the magnetic memory has a high integration degree and consumes a reduced power.
Other objects, features and advantages of the present invention will be obvious from the following description.